4.2.4 audio dropout - Mac OS El Capitan - stopped deck/slow waveform

Hi, I've upgraded my Macbook (Mid 2012, 2.5ghz i5 - 8gb RAM) from Mountain Lion to Mac OS X El Capitan and (also SATA to SSD upgrade) and since upgrading I've played 2 gigs :

first one was with 2xCDJ900 (latest firmware) in HID mode - after some hours playing I received some audio dropouts; Rekordbox became unusable so I was lucky to have my backup USB keys with me

second one was with1CDJ2000NXS2. & 1CDJ2000NXS in HID mode (also latest firmware) I've had an audio dropout again while the CDJ2000NXS2 was playing, but was able to continue on the CDJ2000NXS and afterwards I've had no issue.

I also was able to see that the waveform while playing on first deck became slower & choppy.  Audio dropout meter was only at 10% so no CPU problem...

Software problem ? Please assist!
Otherwise I think I will just downgrade to Mac OS Mountain lion again...